Oil, at $4.00 US dollars a gallion, is $34.80 for 1 million BTUs, at 82% efficiency.
Nat Gas at $3.25 a Therm, is $34.20 for 1 million BTUs, at 95% efficiency.
Although oil is higher then that in my area. NG is only $1.71 per therm in my area. Nat gas has to have a very high price before it cost more to use then oil.
100,000 BTUs, weather generated by oil or gas, heats the same, at the same outdoor temps.
That "Gas does not heat as well" line is an old oil company misleading sales tactic.